1. Field of the Invention
The present invention relates to a method and an apparatus for decoding image data having been coded in a multi-structure bit stream format according to a method such as wavelet transform or DCT for a JPEG file. The present invention also relates to a computer-readable recording medium storing a program to cause a computer to execute the image decoding method.
2. Description of the Related Art
In the field of data compression, such as image data compression by an image server of a medical network or general data compression in communications or filing, various algorithms have been proposed. For example, as efficient compression algorithms, a WTCQ method (P. Siram and M. W. Marcellin, xe2x80x9cImage coding using wavelet transforms and entropy constrained trellis-coded quantizationxe2x80x9d, IEEE Transactions on Image Processing, vol. 4, pp. 725-733, June 1995) or a SPIHT method (A. Said and W. A. Pearlman, xe2x80x9cA New Fast and Efficient Image Codec Based on Set Partitioning in Hierarchical Treesxe2x80x9d, IEEE Transactions on Circuits and Systems for Video Tech., vol. 6, pp. 243-250, June 1996) have been proposed. In these methods, original image data representing an original image are subjected to a wavelet transform and transformed image data at multiple resolutions are obtained. The transformed image data are then classified and bit allocation is determined. According to the bit allocation having been determined, quantized data are obtained by quantization using a TCQ method. The quantized data are then subjected to entropy coding and coded image data are generated.
In the coding method using the wavelet transform, the original image data are converted into image data in a multi-resolution space representing an object in a plurality of frequency bands, and the coded image data have a multi-structure bit stream format.
In DCT used in the field of general JPEG compression, original image data are divided into blocks and subjected to discrete cosine transform (DCT). Bit allocation is then determined and quantization is carried out on the transformed data according to the bit allocation. The quantized data having been generated in this manner are then coded to generate coded data. In the coding method using DCT, data in each block are decomposed into a direct current component and an alternating current component. The alternating current component represents an object from low to high frequencies in each block. Therefore, the coded image data have a multi-structure bit stream format.
An object of the present invention is to provide a method and an apparatus for decoding, in a desired mode, coded image data having a multi-structure bit stream format as has been described above and also to provide a computer-readable recording medium storing a program to cause a computer to execute the image decoding method.
An image decoding method of the present invention is an image decoding method for obtaining decoded image data by decoding coded image data having been generated by coding image data in a multi-structure bit stream format, and the image decoding method is characterized by that the coded image data are decoded by changing a degree of decoding.
xe2x80x9cCoding image data in a multi-structure bit stream formatxe2x80x9d means coding after image data have been converted into a multi-resolution space by a wavelet transform or DCT as has been described above.
xe2x80x9cChanging a degree of decodingxe2x80x9d means decoding enabling reproduction up to a highest resolution for a high-quality image in the multi-structure bit stream format, or decoding up to an intermediate resolution in order to shorten an operation time for decoding at the cost of image quality sacrifice to some degree. In the case of decoding up to an intermediate resolution, enlargement by using an interpolation operation to have the same size as the original image data is included as xe2x80x9cdecodingxe2x80x9d in the present invention.
It is preferable for the image decoding method of the present invention to enable selection of a plurality of decoding modes predefined in accordance with the degree of decoding so that the coded image data can be decoded in accordance with one of the decoding modes having been selected.
Furthermore, it is preferable for one of the decoding modes to define the degree of decoding based on the operation time for decoding and further to define the degree of decoding based on the quality of an image obtained by reproduction of the decoded image data.
Moreover, it is preferable for one of the decoding modes to define the degree of decoding based on the kind of output means for outputting the decoded image data. Furthermore, it is also preferable for one of the decoding modes to define the degree of decoding based on usage of the decoded image data.
An image decoding apparatus of the present invention is an image decoding apparatus for obtaining decoded image data by decoding coded image data having been generated by coding image data in a multi-structure bit stream format, and the image decoding apparatus comprises:
decoding means for decoding the coded image data by changing the degree of decoding.
It is preferable for the image decoding apparatus of the present invention to further comprise selection means for selecting a plurality of decoding modes predefined in accordance with the degree of decoding. It is also preferable for the decoding means of the present invention to decode the coded image data according to one of the decoding modes selected by the selection means.
Furthermore, it is also preferable for one of the decoding modes in the image decoding apparatus of the present invention to define the degree of decoding based on an operation time for decoding. Moreover, it is also preferable for one of the decoding modes to define the degree of decoding based on the quality of an image obtained by reproduction of the decoded image data.
It is also preferable for one of the decoding modes to define the degree of decoding based on the kind of output means for outputting the decoded image data. Furthermore, it is also preferable for one of the decoding modes to define the degree of decoding based on usage of the decoded image data.
The image decoding method of the present invention may be provided as a computer-readable recoding medium storing a program to cause a computer to execute the method.
According to the present invention, when coded image data in a multi-structure bit stream format are decoded, the degree of decoding can be changed. Therefore, coded image data can be decoded to a desired degree of decoding, in response to a request upon decoding. For example, when reduction in the operation time is necessary, the degree of decoding is changed not to decode to the highest resolution. When a high-quality image is desired, the degree of decoding is changed to decode up to the highest resolution.
Moreover, by enabling selection of the decoding mode from the predefined decoding modes, the degree of decoding can be changed easily, which improves convenience for users.